Let x and y be real numbers such that
2<x−yx+y<5.
If xy is an integer, what is its value?
See this link: https://www.wolframalpha.com/input/?i=Give+integer+solutions+++++%5B++2+%3C+%5Cfrac%7Bx+-+y%7D%7Bx+%2B+y%7D+%3C+5+%5D
Let xy=z.2<x−yx+y<52<z−1z+1<52z+21<z−12<5z+51:2z+2<z−1z<−32:z−1<5z+5−6<4zz>−32Combine 1 and 2:−32<z<−3The only integer solution is z=−2∴xy=−2
.Note that x-y must be larger than x+y if the ratio is to be between 2 and 5. This will be the case if y is negative.
So we have (x-y)/(x+y) > 2, or. x-y > 2x+2y or x < -3y or x/y > -3 where the < becomes > because we are dividing by a negative number.
We also have (x-y)/(x+y)<5, or x-y < 5x+5y or 4x > -6y or x > -(3/2)y or x/y < -3/2 where > becomes < because we are dividing by a negative number.
Hence -3 < x/y < -3/2, giving x/y = -2
Max got the right answer with incorrect reasoning! It happens!